RF power amplifiers are used in a wide variety of communications and other electronic applications. These amplifiers are made up of one or more cascaded amplifier stages, each of which increases the level of the signal applied to the input of that stage by an amount known as the stage gain. Ideally, the input to output transfer of each stage is linear; a perfect replica of the input signal increased in amplitude appears at the amplifier output. In reality, however, all power amplifiers have a degree of non-linearity in their transfer characteristic. This non-linearity results in the distortion of the output signal so that it is no longer a perfect replica of the input. This distortion produces spurious signal components known as intermodulation products. Intermodulation products are undesirable because they cause interference, cross talk, and other deleterious effects on the performance of a system employing the amplifier. Accordingly, the prior art reflects various methods and devices designed to reduce the distortion produced during a power amplifier's operation. Two methods commonly suggested are predistortion and feedforward.
Predistortion utilizes an auxiliary distortion source that produces an auxiliary distortion signal similar to the distortion generated by the power amplifier. The auxiliary distortion signal is added to the power amplifier input in the correct gain and phase to promote cancellation of the distortion at the power amplifier's output. This method requires matching the distortion characteristics of two dissimilar sources and hence limits the amount of correction which can be obtained.
The feedforward method does not have this limitation because it separates out the distortion generated by a power amplifier and adds it back into the power amplifier's output with gain, phase and delay adjusted for maximum cancellation. The amount of distortion reduction available using feedforward is primarily limited by the accuracy of the gain and phase adjustments.
Referring to FIG. 1, a prior art feedforward system is shown in block diagram form. Splitter circuit 105 divides the input signal 100: one part is sent to power amplifier 110 and the other to cancellation circuit 130 via path 106. The output from power amplifier 110 includes a distortion component caused by the amplification of the input signal 100. A portion of the output signal from the power amplifier 110 is taken from directional coupler 115 and sent to cancellation circuit 130. The gain, phase and delays, of the input signal on lead 106 is adjusted by fixed gain and phase 135 and delay 140 adjusters so that a portion of the input signal is canceled when combined with the signal from directional coupler 115, to derive a distortion component on lead 131. The distortion component is adjusted by fixed gain and phase 145 and delay 150 adjusters, so that when the distortion component is combined with the power amplifier output, at directional coupler 125, the resultant output signal 160 is free from distortion. The problem with this method, however, is the use of fixed gain, phase and delay adjusters which preclude the ability to adjust gain and phase parameters in response to operating point changes, such as, for example, input signal variations, voltage variations, and temperature fluctuations.
Referring to FIG. 2, there is shown yet another prior art feedforward system which attempts to overcome the above mentioned shortcomings. For convenience similar elements retain the same numbering as in FIG. 1. A test signal, or pilot 209, is injected, via coupler 208, into the main signal path of power amplifier 110. The magnitude of the pilot 209, when detected at the amplifier output by receiver 262 via coupler 261, is used by automatic control circuit 263 to adjust the gain and phase of signals on lead 242 in order to eliminate both the pilot and the distortion introduced by the power amplifier 110. The problem with this approach is that the injection of a single pilot tone fails to provide a wide-bandwidth solution to intermodulation product cancellation. In addition, the embodiment in FIG. 2 still teaches the use of fixed gain, phase and delay adjusters to provide carrier cancellation.
A further feedforward amplifier system is known from U.S. Pat. Nos. 5,077,532 to Obermann et al., and 5,130,663 to Tattersall, Jr., both assigned to the same assignee of the present invention and incorporated herein by reference. Both of these patents describe circuitry for use in automatically aligning feedforward amplifiers, the latter using a pilot tone signal injected before the main amplifier to further aid in determining the IM distortion. However, feedforward circuits such as disclosed in these patents are not as advantageous for use with type 2 feedforward amplifiers (i.e., one in which the error amplifier handles some amount of the wanted signal power), because the carrier power detector is at the input or output of the error amplifier, and the carrier cancellation adjustment comes before the error (pilot) cancellation adjustment.
It would be highly advantageous therefore to provide a feedforward distortion minimization circuit that continuously, accurately and efficiently performs the gain and phase adjustments necessary to improve and maintain the intermodulation performance of a power amplifier, while avoiding unwanted interactions between cancellation adjustments and while minimizing power wasted in a dump load following the output combiner.